Be they anchovies, sardines, or krill, schools of sea critters all take the same unusual shape, new research shows.

That shape, resembling a "multifaceted lozenge," results from a competing drive by the animals to stay in the middle of the pack to avoid being eaten while needing to seek the edges for a breath of oxygen from time to time, the authors say.

"It's a little like being in the middle of a hot, sticky nightclub," said Andrew Brierley of the Gatty Marine Laboratory at the University of St. Andrews in Fife, Scotland, who conducted the study with Martin Cox. "You need to nip outside for air."

"It doesn't take long for the middle of an aggregation to get depleted of oxygen," Brierley said, based on the pair's analysis, published online today in Current Biology.

The researchers gathered data on the 3-D shape of krill swarms using specialized sonar and analyzed characteristics of the shoals. They found no uniformity in shoal size or density, but when they looked at the ratio of the surface area to the volume of the shoal, it was constant.

What's more, when they analyzed measurements of anchovy and sardine schools collected and published by other researchers, the fish schools showed the same ratio.

"This is very surprising," said François Gerlotto of the Research Institute for Development (IRD) in Sète Cedex, France, who called the work "beautiful." "I would not have imagined that they have the same shape."

Brierley's team was also surprised to find the common factor, especially across both schooling fish and krill, which are tiny, shrimp-like crustaceans.

"You never get uniform relationships like that in biology," he said.

To try to explain the common shape, the team created computer models of shoals to see how different hypothetical behavioral rules affected the resulting shape.

The team got the best prediction for the observed shape when they incorporated only the drive to avoid predators by staying near the center and the need to get oxygen near the edges.

"It's a simple tradeoff between a shape that provides a good number of organisms with the protection of an aggregation, but a surface area that's large enough to give space at the edge for individuals to get out to get a breath of fresh air," Brierley said.

Incorporating other postulated reasons for schooling such as mating or finding food made the shape predictions worse. "You don't get as good a fit that's as good as if you just use oxygen and predator avoidance," he added.

The importance of oxygen in the team's findings is convincing, Gerlotto said. "Oxygen is now recognized as a key parameter in the ocean for fish distribution and survival," he said.

"I am not completely convinced by the predation stuff," he added.

Large schools may not actually offer protection from predators, he said, because they may be easier for predators to find. They may create a detectable wake and emit an easy-to-detect smell from waste or other molecules given off by all members of the group.

The importance of oxygen could have serious implications for the future, because warmer seas can hold less dissolved oxygen. Gerlotto has observed that after El Niño years, when waters warm, jack mackerel schools are smaller than in other years.

"If we have changes in the global temperature, we will have dramatic changes in the fish distributions. That means the marine ecosystem will be different," Gerlotto said.

These shoaling organisms are important because they serve as a key source of food for marine animals up the food chain. "If there is any effect on these fish, it will have consequences," he said.